The measurement of the amount of water in a solid or gas system is often considered an easy test to perform. However, the accurate measurement of relatively small amounts of water or the measurement of the amount of a water in a relatively small sample is difficult to achieve. The determination of the amount of water in a small sample is of particular importance in industries which produce pharmaceutical products, biological products or microchips, as well as the transportation of materials such as natural gas. In all of these areas, moisture content can affect the shelf life biological activity or the equipment employed during transportation and handling.
A number of assay methods are known which can determine the moisture content of a sample. However, the known assay methods are subject to variation in results and are unable to accurate assay the relatively small amounts of moisture in relatively small samples. Often, due to the sensitivity limitations of the assay method, a number of samples must be combined and the resulting measurement is only an estimated average of the moisture actually presented in each individual sample.
The oldest method for ascertaining the moisture content of a material is the loss in weight method. In the loss in weight method, a material sample, typically from 300 to 500 mg, is placed in a vacuum desiccator containing a desiccant such as phosphorous pentoxide maintained at ambient temperatures. The pressure in the desiccator is reduced to less than 1 Torr and the sample weighed at various time intervals until a constant weight is achieved. The loss in weight is attributed to that of the moisture in the materials. Difficulties arise due to the requirement that a sample of from about 300 to 500 mg be employed in order to provide an acceptable margin of error as well as the possibility of inaccurate weighing measurements, lack of effectiveness of the desiccant and problems of controlling moisture changes in the sample during the actual weighing process. Also, the loss in weight technique is not generally suitable for determining the moisture content of gases.
Another method for determining moisture content is commonly referred to as the Karl Fischer method. This method has been automated for routine use in an apparatus available from a number of sources. This method involves the solubility of the sample in methanol and a reaction and titration with an iodine based reagent. Automated machines typically employ a conductometric endpoint detection circuit. The volume of the sample required and the dryness of the extractant limit the lower detection limits to moisture values to from 5 to 10 parts per million. Typical commercial autotitrators which measure water by this method have an accuracy of about +or -10 micrograms of water. Thus, relatively large samples must be employed in order that the experimental error inherent in the process be held within acceptable limits.
An electrolysis method of moisture measurement is also known. The electolysis method is often referred to as the Du Pont method. In this method, the sample is placed in a chamber and gas such as dry nitrogen is passed over the sample. The water vapor from the sample is carried to an analytical chamber by the dry gas where the moisture reacts with a film of concentrated phosphorous pentoxide to form phosphoric acid. The phosphoric acid is quantitatively electrolized by electrodes embedded in the film. The electrolitic current is directly proportional to the mass flow rate of water through the cell. This method cannot be employed for a gas stream that may at times reach saturation. Also, hydrocarbons in the sample may coat the surface in the analytical chamber resulting in inaccurate readings.
The vapor pressure method is also a known method for determining the moisture content of a sample. In the vapor pressure method, a sample is placed in a flask connected to a vacuum pump by a U-shaped vapor trap. The vapor trap is cooled to a temperature of approximately -50.degree. C. by a dry ice in alcohol mixture. As the system is evacuated by a vacuum pump, water vapor from the sample is collected in the trap. The vapor trap is then isolated and allowed to warm to ambient temperatures. The increase in pressure is a measurement of the moisture removed from the sample. With this process, inaccurate results may be obtained due to the water vapor passing through the trap without condensing. This is especially so at higher pressures where gas flow is viscous rather than molecular. Also, at the temperature of dry ice and alcohol of approximately -55.degree. C., the vapor pressure of water vapor of ice is approximately 29 Torr. Thus, sublimation of the ice formed in the trap may result in water vapor being lost through the vacuum pump resulting in an inaccurate determination.